In known configurations, (for instance, Japanese Patent No. 2783945) such a conventional smoke sensor A comprises, for instance, a housing 20, an LED (light-emitting section) 6, and a photodiode (light-receiving section) PD, as illustrated in FIG. 30(a). In the smoke sensor A disclosed in Japanese Patent No. 2783945, the LED 6 outputs light intermittently towards a sensing space within the housing 20, while the photodiode PD, which is disposed at a position on which direct light from the LED 6 is not incident, converts the received light into current. When smoke flows into the sensing space of the smoke sensor A, the smoke gives rise to diffusion and reflection of light from the LED 6 within the sensing space. This causes an increase in the amount of light, from the LED 6, that is received by the photodiode PD, and thereby increases the amount of current outputted by the photodiode PD.
The LED 6 and the photodiode PD comprise a projection lens 23 disposed in front of the LED 6, a light-receiving lens 24 disposed in front of the photodiode PD, and an optical block 25. The housing 20 comprises a body 26 and a cover 27. The body 26 houses the optical block 25, which has an opening in its lower face, in such a manner that light from the LED 6 exits towards the opening. The cover 27 has a bottomed tubular shape, with an opening at its top face, and is joined to the body 26 so as to cover the opening of the body 26. An opening window for smoke intake is formed in the peripheral wall of the cover 27. The sensing space is formed inside the cover 27. An insect screen 28 that prevents the intrusion of insects into the sensing space, and a labyrinth 21 that prevents ambient light from being incident into the sensing space, are disposed inside the cover 27, so as to surround the sensing space. The labyrinth 21 uses a complex structure having convoluted optical paths in order to prevent intrusion of various types of ambient light, for instance from fluorescent lamps, incandescent lamps or the like, and to prevent the photodiode PD from being struck by light from the LED 6 in a state where no smoke is in the sensing space.
In such a smoke sensor A, a current-voltage conversion circuit (IV conversion circuit) 2 that converts an input current, from the photodiode PD, into voltage, and that outputs the voltage, is provided in a circuit block 1 housed in the housing 20, as illustrated in FIG. 30(b). The smoke sensor A is configured in such a manner that the output voltage from the current-voltage conversion circuit 2 passes through an amplifier circuit 12 and a filter circuit 13, is inputted to a alarm set-off determination circuit 14 that is a determination processing section, and an alarm is set off by the alarm set-off circuit 15 (buzzer or the like) when the amount of change of the output voltage exceeds a predefined fire determination level. The circuit block 1 comprises a power source circuit 16 that supplies power to the various circuits, a driving circuit 17 that drives, for instance, other alarm set-off means, and a LED driving circuit 18 that causes the LED 6 to periodically emit light in pulses. The LED driving circuit 18 comprises a transistor Tr1 (FIG. 31) connected in series with respect to the LED 6.
The current-voltage conversion circuit 2 used herein has a conversion section 3 that comprises an operational amplifier OP1, for instance as illustrated in FIG. 31. In the conversion section 3, a converting resistor R2 is connected between an inverting input terminal and an output terminal of the operational amplifier OP1. The conversion section 3 is configured in such a manner so as to output, at an output terminal Tout, an output voltage V10 whose value fluctuates according to fluctuation of an input current I20 that is inputted to the inverting input terminal. In the example of FIG. 31, a reference voltage Vs is applied to a non-inverting input terminal. Therefore, the output voltage V10 is represented by V10=Vs−(I20×r2), wherein r2 is the resistance value of the converting resistor R2. The current-voltage conversion circuit 2 causes the output voltage V10 to fluctuate with reference to an operating point, in accordance with the fluctuation of the input current I20, taking, as the operating point, the output voltage V10 at a steady state where the photodiode PD is not receiving light from the LED 6.
Thanks to their straightforward installation, recent years have witnessed a growing demand for smoke sensors A having batteries as a power source. When using a battery as a power source of a smoke sensor A, the latter must be driven intermittently in order to prolong the life of the battery and curb the average power consumption of the smoke sensor A. In this case, power is supplied intermittently to the current-voltage conversion circuit 2 illustrated in (a) of FIG. 32. Therefore, the LED 6 is driven so as to output pulsed light during the time at which power is supplied to the current-voltage conversion circuit 2, as illustrated in (b) of FIG. 32. When the photodiode PD receives light from the LED 6, upon intrusion of smoke into the sensing space, the amount of change ΔV of the output voltage V10 of the current-voltage conversion circuit 2 becomes greater, and reaches a fire determination level in the figure, as indicated by the solid line of (c) of FIG. 32. If, by contrast, no smoke is present in the sensing space, the amount of change ΔV of the output voltage is small and does not reach the fire determination level, as indicated by the broken line in (c) of FIG. 32.
In a current-voltage conversion circuit 2 such as the one in FIG. 31, a dynamic range of the operational amplifier OP1 is restricted between a power source voltage VDD and a ground GND of the operational amplifier OP1, as illustrated in FIG. 33(a). The above-described output voltage V10 fluctuates as a result within the dynamic range. Therefore, the output voltage V10 becomes saturated when the input current I20 is equal to or greater than a given magnitude.
For instance, the sensing space in the above-described smoke sensor A cannot be completely cut off from the exterior, even despite the presence of the labyrinth 21, and hence ambient light, though little, strikes the photodiode PD. Ordinarily, the ambient light exhibits little fluctuation over time. Thus, when receiving the ambient light, the photodiode PD outputs a current having little fluctuation over time (hereafter, low-frequency component). The output voltage V10 may become saturated in some cases when the low-frequency component comprised in the input current I20 is equal to or greater than a certain magnitude.
Specifically, the operating point of the output voltage V10 becomes the reference voltage Vs, as illustrated in FIG. 33(a), if the input current I20 comprises no low-frequency component. If the input current I20 fluctuates, therefore, the output voltage V10 also fluctuates following the fluctuation of the input current I20. By contrast, the operating point of the output voltage V10 drops, as illustrated in FIG. 33(b), when the input current I20 comprises a low-frequency component. If the input current I20 increases, the output voltage V10 may become saturated halfway through. In particular, the output voltage V10 is brought to a saturated state, regardless of the fluctuation of the input current I20, and the output voltage V10 fails to follow the increase in the input current I20, if the low-frequency component is large and the operating point of the output voltage V10 is lowered down to about ground GND, as illustrated in FIG. 33(c).
For instance, the voltage drop between the terminals of the converting resistor R2 is 1 V for an input current I20 of 1 μA, assuming a resistance value r2 of 1 MΩ for the converting resistor R2, and a reference voltage Vs of 1 V. As a result, the output voltage V10 of the current-voltage conversion circuit 2 becomes saturated, at 0 V. In this state, the output voltage V10 of the current-voltage conversion circuit 2 is saturated, and hence does not fluctuate any further, even if the pulsed input current I20 is inputted to the current-voltage conversion circuit 2 upon reception, by the photodiode PD, of light from the LED 6. In this case, therefore, there is a chance of alarm failure in that the amount of change ΔV of the output voltage V10 does not reach a fire determination level.
Such a smoke sensor A, moreover, may conceivably be configured so as to set off an alarm when an instantaneous value of the output voltage V10 reaches a predefined fire determination level. In this case as well, there is a chance of alarm failure and false alarms when the operating point itself of the output voltage V10 fluctuates due to the influence of the low-frequency component, even if the output voltage V10 is not saturated. An alarm failure occurs when the output voltage V10 does not reach the fire determination level, despite the presence of smoke in the sensing space, and a false alarm occurs when the output voltage V10 reaches the fire determination level, despite the absence of smoke in the sensing space.
In the above smoke sensor A, the labyrinth 27 prevents ambient light from striking into the sensing space, and hence suppresses fluctuation of the operating point of the output voltage V10 due to the influence of ambient light, so that the above-described alarm failures and false alarms are unlikely to occur.
In the above-described smoke sensor A, however, the labyrinth 21 that prevents ambient light from being incident into the sensing space has a complex structure. The manufacturing cost of the labyrinth 21 precludes reducing the costs of the smoke sensor A as a whole. It would therefore be desirable to lower the cost of the smoke sensor A as a whole by simplifying the structure of the labyrinth 21 as much as possible, or by omitting the labyrinth 21 itself.
However, simplifying or omitting the labyrinth 21 results in a greater ambient light intensity being received by the photodiode PD, and in a greater low-frequency component comprised in the input current I20, which causes the operating point of the output voltage V10 to fluctuate. Alarm failures and false alarms become likelier as a result, which is problematic. In the particular case where the smoke sensor A uses a battery as a power source, as described above, the power source voltage of the operational amplifier OP1 is low, and the dynamic range of the operational amplifier OP1 is comparatively narrow. As a result, the output voltage V10 saturates readily.